Communications networks employ fiber optic cables to transport a variety of signals such as voice, video, data and the like. Fiber optic cables were first deployed for long-haul applications and generally used round cable cross-sections. These long-haul cables were designed to meet expected outdoor environmental and mechanical requirements. One example of the mechanical requirements is crush resistance. Simply stated, fiber optic cables in the field experience crush in the form of people walking on them, cars or other vehicles driving over them, having doors closed on them and many other situations where the fiber optic cable experiences crush forces. Generally speaking, because the round fiber optic cables used a generally symmetrical cross-section the crush performance was similar along any cross-sectional orientation.
One generally accepted method for testing fiber optic cable crush performance is to place the cable between two parallel plates and apply a crushing force. The two plates are of known length such as about 100 millimeters long with rounded edges on the plates to keep them from cutting into the optical fiber cable. Either a weight or device such as an Instron laboratory test unit is used to apply a compressive load to the optical fiber cable through the plates for a given time, while keeping the plates parallel. The load applied during testing can depend on the size and application for the fiber optic cable. By way of example, a common load for small optical fiber cables is 1100 Newtons applied for 10 minutes; however, other crush loads and times are possible. While the compressive load is applied, held, and/or removed optical tests, such as attenuation and continuity, are performed on the optical fibers in the cable to determine the optical performance. Additionally, the crushing test normally includes inspections for physical damage to the optical fiber cable and may include measurement of optical fiber cable deformation for analysis since these can also affect performance. One such crush procedure used by the industry is described in TLA/EIA 355.41, FOTP-41 Compressive Loading Resistance of Optical fiber cables.
As the deployment of fiber optic cables continues, the use of non-round fiber optic cable designs is increasing. For instance, as optical fiber cables push toward the subscriber non-round cable designs are being deployed. One reason non-round fiber optic cables are being deployed is because they are easy to install using a pressure type clamp, typically called a P-clamp. U.S. Pat. No. 2,068,368 shows one type of P-clamp for securing a cable to a structure such as a building or a pole in aerial applications. P-clamps are typically used for securing non-round drop cables since it makes the cable deployment efficient, reliable, and cost-effective. The P-clamp grips or clamps the non-round cable along the minor axis of cable. U.S. Pat. No. 6,493,491 discloses a non-round fiber optic cable design 10 like shown in FIG. 1 useful with P-clamps. More specifically, fiber optic cable design 10 uses a cross-sectional area of the reinforcing members 16 that is larger than the cross-sectional area of the cavity 14 for protecting the optical fibers 12 from crushing forces along the direction of the minor axis. The relatively small cavity size also helps crush performance in the direction of the major axis of fiber optic cable design 10. However, the use of a relatively small cavity limits the number of optical fibers within the cavity.
As the size of the cavity increases for non-round fiber optic cables providing suitable crush performance becomes difficult. In other words, designing a non-round fiber optic cable having a relatively large cavity with a reasonable cable cross-section, and suitable crush resistance along both the major axis and minor axis is challenging. The conventional design wisdom for providing acceptable major axis crush performance is to increase the wall thickness of the cable jacket between the cavity and the perimeter of the jacket. Although increasing the jacket thickness improves major axis crush performance, it also has the disadvantages of increasing the cross-sectional footprint along with increased material usage for the fiber optic cable. The present invention is directed to improving the major-axis crush performance of non-round fiber optic cables.